Nozzles are key components of internal combustion engines for organizing and controlling the combustion process. Currently the publicly known nozzles of diesel engines include the following two types: needle type and hole type. Needle type nozzles are used in indirect injection combustion chambers, with which unblocked spray holes are ensured. However, the resulted fuel lines are thicker, and thus the atomization effect is not as good as that of hole type nozzles. As a consequence, needle type nozzles have gradually been substituted by hole type nozzles. Hole type nozzles are used in direct injection combustion chambers, and the resulted atomization quality is better than that of needle type nozzles. However, the spray holes of hole type nozzles are of small diameters, and thus easily blocked during operation. Therefore, hole type nozzles impose a high requirement for fuel quality. Besides, hole type nozzles spray in a manner of liquid columns, which, when injected from a few fixed spray holes, result in relatively large dead angles and a higher penetration level. This leads to the problem of spray-wall interaction (i.e., wetted wall), which is not only hard to overcome, but manifested in uneven fuel distribution and ununiformed sizes of atomized particles, preventing the fuel from being gasified fully and combusted uniformly. This is the main reason why direct injection diesel engines can hardly realize high homogeneous charge compression ignition and low temperature combustion, and thus give rise to high levels of NOX, soot and PM emissions. Additionally, a conical-spray nozzle is also available, whose advantages include high injection speed, fine and uniform atomized particles and macroscopically uniform circumferential spray distribution. A disadvantage of the conical-spray nozzle lies in the fact that much of the kinetic energy of atomized particles is lost due to the impact of the fuel column with a guidance cone, resulting in excessively low spray penetration. For this reason, under medium- or low-load conditions, conical-spray combustion engines exhibit lower specific fuel consumption, lower smoke intensity and lower exhaust air temperature than those of traditional engines, and their performance is deteriorated under high-load conditions. A cross sectional area of a fuel column is extended due to the impact with the guidance cone; when there are too many fuel columns involved in the spray, the fuel columns may interfere with each other, resulting in larger oil particles condensed from oil drops at the intersections of the fuel columns, which leads to insufficient combustion, formation of soot and serious after-burning.
To solve these problems, measures are generally adopted at present such as using finer spray holes, increasing the number of spray holes and employing a high pressure injection. However, three problems that are hard to solve are resulted, as follows:
1. As the diameter of spray holes is further reduced, some spray holes have a diameter of ϕ0.08 mm or even smaller. Too small spray holes are easier to be blocked, while technical difficulty and cost of manufacturing is increase. Furthermore, a more critical requirement for the quality of the fuel is imposed. The reduction of the spray hole diameter is limited by injection duration and penetration rate. In particular, although the vortex is made stronger as the engine spinning speed is increased, the achievable degree of homogenization is actually lowered as the absolute time for producing the gas mixture is shortened. Under the condition of ultra-high-pressure injection, an intense high-frequency pressure oscillation occurs in the pressure chamber, which causes “bubbling” (i.e., cavitation) inside the superfine spray holes. The flow state inside the spray holes is thus influenced, which in turn affects the flow state near the spray holes as well as the atomization of the oil drops.
2. As the number of spray holes is increased substantially (sometimes as many as 17 spray holes), the resulted large number of oil lines may be quite close to each other, causing a relatively higher concentration of fuel at the roots of the oil lines. For the oil lines that are farther apart, they can interference with each other under the effect of the air flow inside the combustion chamber. In some areas, small oil particles may thus be combined and affect the atomization quality, leading to local areas having excessively enriched fuel that aggravates the pollutant emission.
3. Ultrahigh pressure injection is subjected to limitations imposed by the maximum common rail pressure of the fuel supply system. Limited by the strengths of parts involved as well as the driving energy of the fuel pump, ultrahigh pressure injection may cause complexity and dangerousness to the fuel supply system, even to an unbearable extent. An increase of supplementary loss of engine energy may also be resulted.
In order to cope with increasingly severe environmental and energy problems, in recent years attentions have been drawn to studies of combustion theories and techniques to be employed by next-generation internal combustion engines that are most promising for realizing ultra-low or even zero emission. These combustion theories and techniques include homogeneous charge compression ignition (HCCI) and low temperature combustion (LTC). Different from traditional spark ignition gasoline engines and diesel engines that are directly controlled via in-cylinder direct injection, a HCCI engine has a spontaneous ignition combustion process, which is achieved by compressing the gas mixture in the cylinder under both the ignition limit and the stable combustion limit.
Various study models and methods have been adopted in creating conditions for HCCI operation, such as: increasing temperature and pressure in the cylinder by using external and internal heat sources, applying fuel with an especially low octane value, utilizing premixed charge compression combustion and employing variable compression ratio and variable valve timing, etc. However, they share some similar common problems, most eminent of which lies in the difficulty in controlling the ignition time and the combustion rate. As a consequence, HCCI operation under a wide range of spinning speed and across various load conditions is yet to be realized satisfactorily.
Therefore, for realizing satisfactory HCCI, new fuel injection techniques and atomization methods are to be studied and developed to solve the following three problems:
1. How to realize an advanced gas mixture control strategy?
HCCI process is mainly subject to chemical kinetic control of the gas mixture, whose rapid formation is enforced. Hence, typical HCCI fuel atomization of gasoline engines and diesel engines at present generally adopts in-cylinder direct injection. From studies pertaining to HCCI lean premixed combustion and low temperature premixed combustion, it is found difficult for internal combustion engines to realize fully homogeneous gas mixture. It is also found that HCCI is not, and not possible, to be absolutely uniform. This is because gas mixture control is a dynamic control. Even in a static state, fuel particles in a gas mixture premixed outside the cylinder can be naturally subsided, absorbed and combined with each other due to gravity as their mass is greater than that of air molecules. Given that the mass of oil drops is far greater than that of air molecules, the oil drops exhibit irregular turbulent fluctuations under the effect of air flow movement in the cylinder after entering the combustion chamber, and move at a speed far greater than that of the air molecules. The speed of relative movement makes the oil drops separated from the air molecules. Separated by the relative speed, the oil drops, which have a larger mass and thus accelerate faster, may collide, concentrate, absorb and combine with each other at a farther place to form an over-rich area and result in thermal stratification. After entering the combustion chamber, oil drops with higher mass and higher density can penetrate through air molecules with lower mass and lower density to impact on the cylinder wall and an end face of piston. If oil supply is not increased or an air inlet is not heated using an external heat source to increase temperature in the cylinder and facilitate evaporation of oil drops, the internal combustion engine will suffer from low temperature and poor cold start performance, and mixing speed and combustion speed will be lowered significantly. Theoretical analysis as well as a large number of tests have proved that the time needed to burn an oil drop is directly proportional to square of the diameter of the oil drop. Rather large differences between sizes off atomized particles can also significantly influence the uniformity of combustion speed and temperature. Currently, regardless of gasoline engines or diesel engines, air inlet injection or in-cylinder direct injection, there exists a common problem: the fuel atomization is of fuel spray type (i.e., liquid column) injection, which is a passive atomization rather than an initiative atomization. Kinetic penetrating force of oil column type injection of high pressure fuel is concentrated on several fuel sprays, resulting in that fuel distribution and atomized particle sizes are not uniform, that the drawback of formation of oil mist over-rich areas and high temperature areas cannot be overcome for sufficient gasification, and that additional spray-wall interaction (i.e., wetted wall) can be caused easily to generate carbon deposition and diluted engine oil. For oil column type injection of hole type nozzles, numerous studies have demonstrated that even under conditions of ultrahigh pressure and superfine spray holes, mist spray formed by diesel fuel in the combustion chamber is in an oxygen-poor or over-rich state (usually 4 times richer than theoretical stoichiometric ratio). Such anoxic state under high temperature exactly helps generation of polycyclic aromatic hydrocarbons (PAHs), which is the cause of soot generation. Combustion of traditional diesel engines is “diffusive combustion under theoretical equivalence ratio”. According to chemical kinetic theories, combustion flame under theoretical equivalence ratio has the highest temperature, up to 2700K, and is accompanied by the maximal nitric oxide (NOX) generation rate. Therefore, to realize an advanced gas mixture control strategy, the traditional oil column type injection method must be changed.
2. How to solve the problem of HCCI cyclical fluctuation under high compression ratio, high speed and high load?
As HCCI is tended to be rapid combustion and is sensitive to gas mixture temperature and likely to fluctuate cyclically, it is hard to be controlled and is currently limited in low load and medium and-low speed operation areas, rather than high compression ratio, high speed and high load conditions. Therefore, it is necessary to improve the fuel injection method and atomization method to further enhance robustness of HCCI to prevent cyclical fluctuation resulted from alternate knocking and fire.
3. How to solve the problem of atomization time control and accurate ignition?
HCCI of gasoline engines and diesel engines similarly have some common problems, mainly ignition time and combustion speed controls. Due to these problems, HCCI is hard to operate under extensive spinning speeds and loads, and fuel consumption may even be worsened; thus, HCCI cannot meet GB IV (Euro IV) and above laws and regulations. To ensure reliable ignition and combustion control accuracy, various HCCI feedback controls have been discussed and researched, including cylinder pressure sensor, ionic current sensor, bent axle acceleration signal and knocking sensor etc., which are all problematic to some extent. Meanwhile, as atomization time and accurate ignition electronic control system is complex and costly, the difficulty of HCCI engine industrialization may be increased. Therefore, an enforced accurate ignition control means and a reliable low cost solution are needed.
It is currently known that turbine and turboshaft engines use an open centrifugal nozzle or a centrifugal oil flinger, differing from hole type and needle type nozzles used by reciprocating internal combustion engines. As no device for directly opening and closing spray holes is provided inside the centrifugal nozzle or centrifugal oil flinger, spray holes are always open. Structurally, centrifugal nozzles include simple centrifugal nozzles, double oil way double spray opening centrifugal nozzles and double oil way single spray opening centrifugal nozzles. Centrifugal nozzles generally have good atomization performance and large atomization spray cone angle. Hollow umbrella-like oil mists in the centre are easy to mate flow field of air in the combustion chamber. However, centrifugal nozzles have the following drawbacks: (1) For simple centrifugal nozzles, their adjustable range of fuel flow is much narrow under maximum injection pressure drop. As the size of tangential holes is steady, when fuel injection quantity is reduced, the speed of fuel flowing into a spin chamber is certain to be decreased significantly. Consequently, the tangential speed of fuel flowing away from the spray opening can be decreased significantly, thereby leading to serious deterioration of atomization quality. (2) Two independent simple centrifugal nozzles are substantially connected in series for combined operation in a double oil way double spray opening centrifugal nozzle; therefore, the adjustable range of fuel flow is far greater than that of one simple nozzle. However, the double oil way double spray opening centrifugal nozzle has a drawback that when a second main oil way is put into operation in the beginning, atomization quality will be deteriorated in a moment as the starting injection pressure is rather low. (3) Regarding double oil way single spray opening centrifugal nozzles, their adjustable range of fuel flow is much wide. However, their drawback lies in that the two oil ways will interference with each other; due to back pressure, when a second main oil way is put into operation, spinning speed of oil flow in the spin chamber is slowed, so that atomization quality of fuel is seriously deteriorated. Besides, a centrifugal nozzle, whose structure is rather complex and whose tangential holes (grooves) have adjustable area, is further provided. The abovementioned centrifugal nozzles can extend the adjustable range of fuel flow to some extent and properly improve oil atomization quality under low load. However, when the second main oil way is put into operation in the beginning, atomization quality will always be deteriorated obviously and fuel injection quantity will sharply jump in a moment. Meanwhile, the adjustable range of fuel flow, restricted by the variation range of injection pressure, cannot be increased significantly. The spraying, opening and closing of such centrifugal nozzles are achieved by starting and stopping a fuel pump. One or two oil supply pipes are provided between the oil pump and the nozzle. When the oil pump is stopped after the engine is shut down, as spray holes of the centrifugal nozzle are open all the time, fuel from the oil pump to the nozzle will be automatically fully discharged through spray holes from the oil supply pipe and the nozzle, so that the oil supply system will be placed under an oil-free hollow state. When the engine is started again, the fuel pump needs to further fill fuel into space between the empty oil supply pipe and the nozzle so that fuel can reach the spray holes. Fuel pressure is slowly rising in a temporary process. It is lowered instantly when the engine is started, the injection pressure of starting atomization pressure drop is lower than a critical value, thus oil pressure is much low at the beginning of injection. As the centrifugal nozzle starts injecting before fuel pressure reaches a rated pressure value, the atomization and combustion effects are not acceptable, and problems such as exhaust fuming and slow starting response are resulted. Main components of small granular substances contained in the exhaust soot from insufficiently combusted fuel include carbon granules and trace amounts of metal salts etc., which will increase carbon depositions on a flame tube, a combustion box and a turbine blade, leading to lower working efficiency. Carbon deposition can separate metal surfaces of the flame tube, the combustion box and the turbine blade from cold air, causing local overheating in a large area and leading to local heat stress, warping, deformation and cracks. Additionally, carbon deposition can block some nozzles, so that when the engine is operated, non-uniformity of the front temperature field of a turbine is enlarged, and the flame direction is not parallel to the axis of the combustion chamber; therefore, the combustion process of the combustion chamber can be destroyed, and a guidance blade and an operating blade of the turbine can be burnt down to cause accidents. When most nozzles are blocked, the engine can be stalled or automatically stopped, thus endangering air vehicles. Some turboshaft engines adopt a centrifugal oil flinger to supply oil. Centrifugal oil flingers can ensure sufficient atomization of sprayed fuel, and is simple in structure, light in weight and convenient in maintenance. However, the spray holes are likely to produce carbon deposition after the engine has run for a long time and every time when it is started; thus, some spray holes can be blocked, thereby leading to the reduction of oil supply and accordingly lowered engine power. In a serious case, engine speed will oscillate on the ground or cannot reach normal maximum speed. During a flight, when pitch is increased or reduced instantly, the engine cannot recover its constant speed rapidly, so that when a throttle is advanced or retarded, the engine will be vibrated in a pulsatory manner and the body of the engine will be shaken. In a starting process, when 60% to 80% of the cross area of the spray holes is blocked, the oil supply pressure will be severely insufficient, thus the engine speed will be limited. The main cause that carbon deposition is likely to occur to block the present centrifugal nozzles and centrifugal oil flingers is that the spray holes cannot be closed directly, so that after the engine is shut down, when automatically discharged through the spray holes from the oil supply pipe and the nozzle between the oil pump and the nozzle, the fuel is evaporated, decomposed, absorbed and subsided repeatedly for a long time to become hard and thick under the high temperature environment remained in the combustion chamber that has not been cooled yet.
Publication Patent Number: CN818372A is a conical-spray nozzle with a needle valve head protection cover. According to the technical scheme of the patent, a fuel injection method is achieved by enabling high pressure fuel to impact on the head of the needle valve. Due to simple impact injection, some fuel is splashed into the protection cover and oil particles are combined, thus leading to high loss of kinetic energy, low spray penetration and deteriorated combustion under high load. Publication Patent Number: CN201092922Y is a vortex conical-spray nozzle. According to the technical scheme of the patent, high pressure fuel needs to pass through a symmetrical tangential oil feed groove provided along the wall of a pit through an annular gap between the nozzle and an outer circle to form a vortex in the planar pit before it is sprayed through the spray hole at the front end of the nozzle. The fuel flow process has more than one turn, which results in high resistance and weakens the vortex. The planar pit cannot self-clean; thus, too much fuel is remained and permanent carbon deposition is likely to be produced under high temperature. The central hole of the nozzle and the outer circle constitute an interference fit. As their expansion factors are different, the central hole of the nozzle is likely to be loosened and fallen off under high temperature and high pressure. Publication Patent Number: CN2173311Y is a liquid injection atomizing nozzle. According to the technical scheme of the patent, when liquid fuel, under the high pressure of an oil pump, passes through a plurality of spiral grooves that are uniformly distributed on the cylindrical surface of a plunger piston at the lower end of a needle valve, a slant reacting force will be generated upon the spiral grooves to push them to drive the needle valve to rotate reversely, so as to counteract atomization of the spiral grooves. The tooth-shaped cylindrical contact surface of the spiral grooves is not a smooth cylinder, so that movable fit sealing clearance is hard to be ensured between the contact surface and the inner circle of the body of the needle valve. Due to spiral slant injection, the atomization cone angle of fuel is relatively small, the burning centre is forwarded, and the flame is rather long, thus restricting the running load of the engine. The fuel passage of the spiral grooves is long and shallow, so that fuel faces high resistance for flowing and injection. Publication Patent Number: CN1204747A is a return flow type mechanical atomizing nozzle. According to the technical scheme, no needle valve is provided and correct time injection cannot be controlled, so that the nozzle cannot be applied to a reciprocating internal combustion engine. Further, as the spray hole of the nozzle is open and cannot be closed directly, when the nozzle is used in turbine and turboshaft engines and a gas turbine, a problem cannot be solved that fuel is drained from an oil supply pipe after the engine is shut down, thus leading to low fuel pressure at the beginning of injection, affected atomization and combustion effects, exhaust fuming, slow starting response and apt carbon deposition. Publication Patent Number: CN101368740A is a closed pulsatory centrifugal nozzle. According to the technical scheme of the patent, if the diameter of spray holes is increased when a high power heavy-duty engine has high cyclical fuel injection quantity, a fuel film may be thickened and atomization quality can be affected.